Typically, manufacture of colored plastic products involves feeding a mixture of colorant material (e.g., 65/35 wt % colorant/polymer) and neat polymer resin to a single or double screw extruder, mixing in the melt state and molding the melt to a final product form. However, several issues are often encountered with such a process and relate to the initial colorant and polymer materials utilized. For instance, phase separation of the colorant and polymer can lead to a color shift over time in the final product. Such a phenomenon is a direct result of viscosity differential between the two materials. Further, with some colorants or pigments, a “swirling” pattern in the molded product can lead to color distortion. Identification of such issues often requires apparatus shut-down and thorough cleaning, both of which lead to lower production rates and increased cost.
Other processing techniques present particular colorant-related issues. For instance, consider rotational molding, an approach used to produce hollow containers and other products of larger size and complicated structure which cannot be readily molded by other conventional molding processes, such as injection molding and sheet thermoforming. Products produced by rotational molding typically include, for example, gasoline tanks, casks, storage tanks and toys.
In rotational molding, a plastic resin is melted and fused in a closed mold, without application of external pressure. Typically, a resin-charged mold is moved into an oven apparatus and slowly rotated about two axes. As heat penetrates the mold, the resin adheres to the inner surface until completely fused. The mold is then cooled by air and/or water, while still rotating, to gradually lower the internal temperature. Upon removal of the finished part, the mold can be recharged for another process cycle. A variety of polymeric resins can be utilized, including but not limited to polyethylenes, nylons, fluoropolymers, polycarbonates, polypropylenes, polyurethanes and the like. An example of a prior art rotational molding apparatus is found in Friesen U.S. Pat. No. 4,738,815. An example of a prior art polymeric resin powder for use in rotational molding may be found in Inoue et al. U.S. Pat. No. 4,587,318.
As discussed in the Inoue et al. '318 patent, polymeric resin powders used in rotational molding can be mixed with colorants/pigments to impart desired color to the molded product. Such colorants can be added to the polymeric resin powders in the form of dry powders or solids. Unfortunately, these dry materials often present handling and mixing problems. Specifically, dry pigment powders may form undesirable dust or mix unevenly with the polymeric resin powders resulting in non-uniform color distribution in the molded product. Further, colorant additives can agglomerate, leading to poor mechanical performance, and/or “plate-out” the polymer matrix and deposit on an interior mold surface.
In light of such difficulties, various other mixing techniques have been tried. In turbo-blending, for instance, powdered pigments are bonded to the surface of plastic resin powder by rotating both together at high speeds at an elevated temperature. While dispersion is enhanced, many of the same issues remain as with dry mixing (e.g., agglomeration, plating-out and poor mechanical performance, etc.). Such problems can be addressed by compounding a pigment into the polymer resin using extrusion followed by solid-state grinding. While uniform color dispersion can be achieved, high processing temperatures can lead to pigment degradation—especially so with organic colorants. Further, multiple processing steps contribute toward higher operating costs.
As a result, there remains an on-going search in the art—with respect to both general melt processing and, more specifically, rotational molding—for one or more methods to incorporate colorant additives into polymeric resin materials to better utilize the benefits and advantages available from such technologies.